The Earth Science Picture of the Day (EPOD) highlights the diverse processes and phenomena which shape our planet and our lives. EPOD will collect and archive photos, imagery, graphics, and artwork with short explanatory captions and links exemplifying features within the Earth system. The community is invited to contribute digital imagery, short captions and relevant links.

Deceptive Skies

The sky is not always what it seems. It was late afternoon on a chilly mid-March day in the village of Bildeston, Suffolk, England. Surely the solid looking cumulus congestus near the horizon is casting its shadow onto the higher and much brighter cloud bank, is it not? No! The camera was facing in the opposite direction almost towards the Sun, which was to the right and just outside the field of view. And the Sun’s rays almost never shine upwards! So what is happening?

Dense cumulus congestus can tower up to 6km (20,000 ft). Here it is casting its shadow downwards onto thin translucent cloud (possibly altocumulus stratiformis) at lower altitude and closer to the camera. A trick of perspective makes the nearer and lower cloud, together with the shadow projected onto it; appear to be higher in the sky. We see the shadow from the other side of the cloud, just like looking at the back of a movie or cinema screen.

Shadows are in fact three-dimensional. Here in this wider-angle view we see them at left as diffuse shadows coursing (again downwards!) through the hazy air. These shadows are 3D tubes of dark air.

The nearby stratus is so bright because it is thin and its tiny dropletsdiffract sunlight with little deviation mostly forwards towards the camera. The thin edges of the distant cumulus do the same to form the familiar silver lining. There is a hint of color too, cloud iridescence, because the diffraction is wavelength dependent.